Some years before he died, Sydney spoke about the spying secret he had kept for 40 years and about being held at gunpoint by his Chinese captors.

He said: “The guard holding me took his arm from around my neck and stood to one side, still holding onto my hair, keeping me facing the gun muzzle.

“The gun muzzle was placed about an inch from my right eye. It was too close for my eyes to focus on, but I clearly saw his right hand and his finger on the trigger.

“I never got used to it. It never failed to unnerve me and I always tried not to show it, keeping up that pretence of the British stiff upper lip which we British know is simply suppressed hysteria.”

Sydney considered himself lucky that he was not shot. His story was told in a book, Spy on the Roof of the World.

He was in the bar of the Bryn Tyrch Hotel in Capel Curig in 1954 when he dreamed of organising the first Welsh Himalayan expedition. The trip came at a time when there was mounting evidence of a Chinese military build-up in Tibet.

The Indian Military Intelligence decided to find out more, and in London, Sydney was recruited to report on Chinese military activity in the region. He chose the codename Conway. He did not reveal that the expedition would also involve spying on the Chinese.

He was relentlessly interrogated, threatened at gunpoint and imprisoned in appalling conditions. He did not expect to survive the ordeal. He expected to be imprisoned for a long time by the Chinese and that eventually he would die from dysentry unless he was given medical aid which depended on him signing a confession that he was a “western fascist lackey”.

But as international pressure mounted, the Chinese set Sydney and his two companions free after six weeks of terror.

They were abandoned at the foot of a Himalayan pass never before climbed in winter. He said the Chinese expected him and his colleagues to die. Sydney had with him the notes he had written on bits of toilet paper, even chocolate wrappers from the last of his high altitude rations.

He finally arrived home suffering from frostbite, dysentery and malnutrition.

He followed his interest in underwater archaeology but never climbed a mountain again.